1. Field of the Invention
The present invention relates to a sputtering target for use in forming a Cu—In—Ga—Se quaternary alloy film for forming a light absorbing layer of a solar cell, and a method for producing the same.
2. Description of the Related Art
In recent years, thin film solar cells made by using a compound semiconductor have been practically employed. The thin film solar cell made by using the compound semiconductor has a basic structure in which an Mo electrode layer serving as a positive electrode is formed on a sodalime glass substrate, a light absorbing layer consisting of a Cu—In—Ga—Se quaternary alloy film is formed on the Mo electrode layer, a buffer layer consisting of ZnS, CdS, and the like is formed on the light absorbing layer consisting of the Cu—In—Ga—Se quaternary alloy film, and a transparent electrode layer serving as a minus electrode is formed on the buffer layer.
As a method for forming a light absorbing layer consisting of the Cu—In—Ga—Se quaternary alloy film described above, a vapor deposition film-forming method is known. Although a light absorbing layer consisting of a Cu—In—Ga—Se quaternary alloy film, which has been obtained by the method, may exhibit high energy conversion efficiency, the vapor deposition film-forming method attains slow speed for forming a film. Hence, when a large area film is formed, the uniformity of the in-plane distribution of the film thickness may not be ensured. Therefore, a sputtering method for forming a light absorbing layer consisting of a Cu—In—Ga—Se quaternary alloy film has been proposed.
As a sputtering method for forming a Cu—In—Ga—Se quaternary alloy film, a method (so called “selenization method”) has been proposed in which a Cu—Ga binary alloy film is firstly formed by sputtering using a Cu—Ga binary alloy sputtering target, an In film is formed on the Cu—Ga film by sputtering using an In sputtering target, and a film stack consisting of the obtained Cu—Ga binary alloy film and In film is subject to heat treatment in a Selenium and Sulfur atmosphere to thereby form a Cu—In—Ga—Se quaternary alloy film (see Patent Documents 1 and 2).
On the other hand, in order to improve the power generation efficiency of a light absorbing layer consisting of a Cu—In—Ga—Se quaternary alloy film, the addition of Na to the light absorbing layer is required. For example, Non-Patent Document 1 proposes the fact that Na content in a precursor film (Cu—In—Ga—Se quaternary alloy film) is typically about 0.1%.